Dittrich E, Haft C R, Muys L, Heinrich P C, Graeve L
Institute of Biochemistry, Rheinisch-Westfälische Technische Hochschule Aachen, 52057 Aachen, Germany.
J Biol Chem. 1996 Mar 8;271(10):5487-94. doi: 10.1074/jbc.271.10.5487.
The interleukin-6 (IL-6) receptor complex is composed of two different subunits, the IL-6 binding protein (IL-6R, gp80) and the signal transducing component gp130. Our previous studies revealed that the 10-amino acid sequence TQPLLDSEER within the intracellular domain of gp130 is crucial for the efficient internalization of IL-6. Since this sequence contains a putative di-leucine internalization motif, we further analyzed this region by constructing two additional deletions and a series of point mutants. Analyses of these mutants showed that the di-leucine pair (Leu-145 and Leu-146) is essential for ligand internalization, with leucine 145 being less resilient to exchanges. Furthermore, when a chimeric protein (Tac-STQPLL) composed of the Tac antigen fused to the hexapeptide STQPLL of gp130 was studied, we found that this sequence is sufficient to mediate endocytosis and lysosomal targeting of the chimera. Mutational analysis of three serine residues upstream of the di-leucine motif revealed that mutation of serine 139 to an alanine reduces the initial internalization rate by 50%. This finding suggests that a serine phosphorylation may be important for rapid endocytosis.
白细胞介素-6（IL-6）受体复合物由两个不同的亚基组成，即IL-6结合蛋白（IL-6R，gp80）和信号转导成分gp130。我们之前的研究表明，gp130胞内结构域内的10个氨基酸序列TQPLLDSEER对于IL-6的有效内化至关重要。由于该序列包含一个假定的双亮氨酸内化基序，我们通过构建另外两个缺失突变体和一系列点突变体进一步分析了该区域。对这些突变体的分析表明，双亮氨酸对（Leu-145和Leu-146）对于配体内化至关重要，其中亮氨酸145对交换的耐受性较低。此外，当研究由与gp130的六肽STQPLL融合的Tac抗原组成的嵌合蛋白（Tac-STQPLL）时，我们发现该序列足以介导嵌合体的内吞作用和溶酶体靶向。对双亮氨酸基序上游的三个丝氨酸残基进行的突变分析表明，将丝氨酸139突变为丙氨酸会使初始内化速率降低50%。这一发现表明丝氨酸磷酸化可能对快速内吞作用很重要。
